The present invention is directed to the field of premix combustion systems, particularly those of the type with reduced emissions.
Premix is obtained through the mixing of fuel and oxidant prior to introducing the premix to the source of ignition stabilization. As the fuel and oxidant content of premix approaches uniformity, i.e. homogeneity, the rate of combustion is primarily determined by the reaction rate of the premix. Premix combustion results in a short flame with rapid energy release, permitting smaller combustion chambers and/or increased burner capacity compared to diffusion combustion.
Premix combustion differs from diffusion combustion in that in diffusion combustion fuel and oxidant are separate until they mix at the flame front. In diffusion combustion, the fuel and oxidant content is less uniform, i.e. less homogeneous, than in premix combustion. In diffusion combustion, the rate of combustion is primarily determined by the mixing rate of the fuel and the oxidant. Because mixing occurs on a time scale that is approximately 104 times longer than reaction rates, diffusion combustion results in a flame that is longer than a premix combustion flame.
xe2x80x9cLeanxe2x80x9d premix indicates a fuel/oxidant mixture containing more oxidant than the amount required to completely combust the fuel. This can be compared to a xe2x80x9crichxe2x80x9d premix that indicates a fuel/oxidant mixture containing less oxidant than what is required to completely combust the fuel. A xe2x80x9cleanxe2x80x9d premix can be indicated mathematically by stating the mixture has an equivalence ratio of less than one. The equivalence ratio ("PHgr") is a normalized way of measuring the proportion of an actual fuel and oxidant ratio compared to the fuel and oxidant ratio required for stoichiometric combustion. This is expressed as:   Φ  =                    (                              Q            Fuel                    /                      Q            Oxidant                          )            ⁢      Actual                      (                              Q            Fuel                    /                      Q            Oxidant                          )            ⁢              xe2x80x83            ⁢      Stoichiometric      
where Q represents an amount, or flow rate, of fuel or oxidant.
It is sometimes desirable to control the emissions of nitrogen oxide compounds (NOx) and carbon monoxide (CO). It has been observed that lean premix having an equivalence ratio of 0.55 less than "PHgr" less than 0.65 (when using natural gas as fuel and air as oxidant) produces very low levels of NOx and CO. As "PHgr" approaches 1.0, NOx increases as flame temperature increases. At stoichiometric combustion ("PHgr"=1.0) fuel and air are in optimum proportion, providing maximum peak flame temperatures. As such, the short, intense premix flame can be hot enough to causing nitrogen and oxygen to react, creating NOx.
While it is sometimes desirable to lower the equivalence ratio to reduce NOx, this can increase CO production. However, as a flame temperature drops in response to a lower equivalence ratio, the flame can become less stable and the flame approaches the lower stability limit. An unstable flame can result in areas of incomplete combustion within the flame, which can produce elevated levels of CO. If the equivalence ratio is lowered further, a lower flammable limit (LFL) is reached, where the heat of combustion is absorbed by the thermal ballast and the combustion reaction cannot be sustained.
The present invention provides an apparatus including a furnace structure in which premix is reacted to form combustion products including an anchor surface having openings through which the premix is introduced into the furnace structure, an array of at least three premix jets, each configured to introduce a corresponding flow of premix into the furnace structure through a corresponding one of the openings, and a supplementary outlet configured to provide anchor combustion products that ignite each corresponding flow of premix introduced into the furnace structure near the openings. Additionally, the present invention provides that the supplementary outlet is further configured to provide anchor combustion products such that each flow of premix is ignited and the ignition distance from the point of ignition of each flow of premix is equidistant from each corresponding one of the openings.
The present invention also provides an apparatus including a furnace structure defining a reaction chamber in which oxidant and fuel are reacted to form combustion products and having an anchor surface having an opening through which the oxidant and fuel are introduced into the reaction chamber. The reaction chamber is configured to recirculate the combustion products back toward the anchor surface. The apparatus also includes an anchor outlet configured to create a layer of combustion products directed along the anchor surface under the influence of the recirculated combustion products. The apparatus further includes a primary outlet configured to direct primary fuel to flow into the reaction chamber through the opening and the layer of combustion products so as to ignite the primary fuel.
The present invention also provides a method including providing a furnace structure defining a reaction chamber having an anchor surface with an opening through which a primary outlet introduces oxidant and fuel into the reaction chamber. The reaction chamber is configured to recirculate combustion products back toward the anchor surface. The method also includes directing anchor fuel to flow from an anchor fuel inlet into the reaction chamber to create an anchor layer that is directed along the anchor surface by the recirculated combustion products. The method further includes directing the primary fuel to flow from the primary outlet into the reaction chamber through the anchor layer.